Multiscale Dynamic Dependence Estimation over Networks

Abstract

In many settings, observed multivariate time series are often nonstationary in nature, i.e., their second order properties vary over time. An additional feature is that their cross-channel dependencies are structured by an underlying network. Together, they give rise to complex interactions between temporal dynamics and network topology. We propose Locally Stationary Wavelet processes on Networks (Net-LSW), a new framework for modelling multiscale, time-varying dependencies that explicitly incorporates the network structure. Unlike traditional multivariate approaches, the Net-LSW process encodes the graph directly in the covariance structure of its random increments. We introduce the concept of local partial correlation graph, mathematically connecting absent edges to zero entries in the time-scale dependent inverse wavelet spectral structure. For inference on the local cross-nodal (partial) dependence, we develop a novel subprocess-based estimation scheme and establish its consistency properties. This new pipeline for network-based nonstationary process modelling, complete with estimation and simulation capabilities that extend outside time-varying vector autoregressive models, is shown to accurately recover evolving dependence structures whilst respecting the underlying graph topology. The analysis of daily stock price volatilities across a global bank network captures multiscale, highly nonstationary dependencies and identifies time-varying systemic shifts during major financial shocks, including Brexit and the COVID-19 pandemic.